The present invention relates to the field of integrated circuits and integrated circuit manufacturing, and more particularly, to making interconnection structures with enhanced electromigration resistance, and while not significantly increasing the resistivity of the metal.
A metal interconnect system in wide use in the later 1990""s included an Al+Cu alloy interconnect line clad on each side with a barrier metal, and combined with planarized tungsten plugs for vias. A via is the structure that provides the electrical connection from one vertical level of interconnects to the next. The system saw wide acceptance in the industry, especially for high performance logic applications, such as microprocessor chips. The system was perceived as satisfactory, except that a severe degradation in electromigration resistance was noted on test patterns with multiple levels of interconnects and tungsten plug vias, versus test patterns using one interconnect level and no vias.
As much as a 100 times reduction in median-time-to-failure (T50) values, or more, were noted. One technical paper covering this phenomenon in detail is by R. G. Filippi et al., entitled, xe2x80x9cThe Effect of Copper Concentration on the Electromigration of Layered Aluminum-Copper (Ti-AlCu-Ti) Metallurgy With Tungsten Diffusion Barriers.xe2x80x9d The paper appears in the 1992 VMIC Conference Proceedings, on page 359. The researchers showed that the copper doping is swept away from the tungsten in the direction of current flow. The aluminum, then depleted of its copper, electromigrates rapidly and voids appear at or near the W/Al interface. Increasing the concentration of copper helps to a limited extent, but degrades the resistivity. Stripes with a close by xe2x80x9creservoirxe2x80x9d of copper also showed improvement, but none of these measures completely solved the problem. In general, the phenomenon may be referred to as a flux divergence at a dissimilar material interface.
A similar phenomenon has been noted in a copper system with tungsten plugs. This was reported, for example, by Kazuhide Abe, et al., and coworkers in a paper entitled, xe2x80x9cCu Damascene Interconnects with Crystallographic Texture Control and Its Electromigration Performance,xe2x80x9d and appears in the IEEE 1998 Reliability Physics Symposium Proceedings on page 342.
The widely-accepted dual Damascene copper systems does not use tungsten plugs between interconnect levels, but does employ a barrier metal. This barrier layer lies, in general, between the upper surface of a copper interconnect and the bottom of an overlying copper via. Thus, some flux divergence may occur at this interface at high current density. The location of the copper metal depletion depends on the direction of current flow. For example, if the current flows up into overlying metal, this is the area of voiding and damage.
In view of the foregoing background, it is therefore an object of the invention to provide a integrated circuit processing method which eliminates or significantly diminishes the flux divergence phenomenon such that little degradation of electromigration resistance occurs at the via structures relative to other regions in the interconnect system.
Another object of the invention is to provide a thin, hardened alloy skin on selected copper surfaces to reduce electromigration resistance and/or provide for passivation.
These and other objects, features and advantages in accordance with the present invention are provided by a method for making an integrated circuit device comprising forming at least one interconnect structure adjacent a substrate by forming at least one barrier layer, forming a doped copper seed layer on the at least one barrier layer, and forming a copper layer on the doped copper seed layer. The method may further comprise annealing the integrated circuit device after forming the copper layer to diffuse the dopant from the doped copper seed layer into grain boundaries of the copper layer. The doped copper seed layer may include at least one of calcium, cadmium, zinc, neodymium, tellurium, and ytterbium as a dopant to provide the enhanced electromigration resistance.
Forming the copper layer may comprise plating the copper layer. In addition, forming the copper layer may comprise forming the copper layer to include at least one of calcium, cadmium, zinc, neodymium, tellurium, and ytterbium as a dopant. In some embodiments, the dopant in the seed layer may be sufficient so that no additional dopant is needed in the copper layer.
The method may further comprise forming at least one dielectric layer adjacent the substrate, and forming at least one opening in the at least one dielectric layer for receiving the at least one interconnect structure therein. Forming the at least one barrier layer may include forming at least one barrier layer comprising metal. The barrier layer may comprise one of tantalum nitride and tantalum silicon nitride. Alternately, the barrier layer may include cobalt and phosphorous. The method may also include forming a displacement plated copper layer on which the at least one barrier layer is formed.
Another aspect of the invention relates to an integrated circuit device. More particularly, the device may include a substrate, at least one dielectric layer adjacent the substrate and having at least one opening therein, and at least one interconnect structure in the at least one opening. The interconnect structure may comprise at least one barrier layer adjacent the at least one opening, a doped copper seed layer on the at least one barrier layer, and a copper layer on the doped copper seed layer. The copper layer may comprise grain boundaries adjacent the doped copper seed layer containing dopant therein. These grain boundaries may be filled during an annealing step during processing. The doped copper seed layer may comprise at least one of calcium, cadmium, zinc, neodymium, tellurium, and ytterbium as a dopant. If desired, the copper layer may also comprise at least one of calcium, cadmium, zinc, neodymium, tellurium, and ytterbium as a dopant.